ANNIVERSARY ADDRESS OF THE PRESIDENT. 7 I 



of Pibbia gneiss. ' They hesitate whether to pronounce it a granite 

 or a gneiss. It exhibits a slight foliation, which has the usual 

 general south-westerly strike ; the larger crystals of felspar, com- 

 monly both in hand-specimens and under the microscope, do not 

 show that definiteness of outline which is characteristic of normal 

 granites, and the ground-mass also differs in similar respects. At 

 the same time we occasionally find places where the rock has the 

 aspect of a normal granite ; moreover, on examining in the British 

 Museum the collection of the rocks pierced by the tunnel, I have 

 been unable to identify this Fibbia gneiss. Yet, on the St. Gothard 

 Pass, it occupies a belt about 2000 yards wide, measured across the 

 supposed strike, and the line of the tunnel is not more than a couple 

 of miles distant. Hence I am now inclined to regard this " Fibbia 

 gneiss " or " Gothard granite " as a true intrusive granite which has 

 been subsequently modified by pressure. It differs, I may observe, in 

 many marked respects from the more porphyritic varieties of the 

 granitoid gneiss already described iu the glen of the Heuss, and has 

 points in common with sundry masses in the more southern part 

 of the Alps which are generally admitted to be intrusive. In 

 descending from the pass on the southern side, we pass over, first, a 

 narrow zone of rather granitoid gneiss, the relations of which to 

 the first-named I have never succeeded iu satisfactorily deter- 

 mining*, and then, at a height of perhaps 1500 feet above Airolo, we 

 find some remarkably well-banded gueiss of rather finely crystalline 

 texture, dipping at a high angle to the jST.W., and so retaining the 

 old strike. In one set of bands quartz and felspar predominate, in 

 the other mica, chiefly biotite ; these bands not seldom exceed an 

 inch in thickness, and I cannot account for them on any other theory 

 than that of some kind of stratification. I may, however, add that 

 the rock had a very compressed look, and recalled to my mind certain 

 sediments (like some at Torcross) which have been exposed to great 

 pressure in a direction at right angles to the bedding. Below this, 

 at no great distance, a series of well-marked schists sets in, and 

 extends down the slopes to a little above Airolo. They are chiefly 

 mica-schists and, as a rule, are distingushed by the conspicuous 

 presence (though in minute individual scales) of a white mica 

 (paragonite), which gives a peculiarly silvery and silky aspect to the 

 apparent surfaces of foliation. These mica-schists exhibit much 

 variation, they are frequently rich in garnet (the crystals being some- 

 times nearly an inch in diameter), and not rarely actinolitic (the 

 crystals being occasionally two or three inches long) ; occasionally 

 also the schists are chloritic, and assume a pale dull green tint ; now 

 and then we find a band of massive dark hornblende-schist. 

 Without relying upon the presence of the last, and making every 

 allowance for the effects of subsequent pressure (which have doubt- 

 less been very considerable), I cannot explain the association of the 

 different members of these schists, often very well-marked bands, 

 except by original differences of mineral composition, i. e. b}' some 



* At my last visit the sections in tins part were much obscured by the un- 

 usual amount of snow. 



